BEIJING – China aggressively deterred dissent in the capital on Thursday's 20th anniversary of the crackdown on democracy activists in Tiananmen Square. But tens of thousands turned out for a candlelight vigil in Hong Kong to mourn the hundreds, possibly thousands, of demonstrators killed.

The central government ignored calls from U.S. Secretary of State Hillary Rodham Clinton and even Taiwan's China-friendly president for Beijing to face up to the 1989 violence.

In Beijing, foreign journalists were barred from the vast square as uniformed and plainclothes police stood guard across the area, which was the epicenter of the student-led movement that was crushed by the military on the night of June 3-4, 1989.

Security officials checking passports blocked foreign TV camera operators and photographers from entering the square to cover the raising of China's national flag, which happens at dawn every day. Plainclothes officers confronted journalists on the streets surrounding the square, cursing and threatening violence against them.

Tourists were allowed in Tiananmen as usual, though security officials — paramilitary, police and plainclothes officers — appeared to outnumber the visitors.

The repression on the mainland contrasted starkly with Hong Kong, where tens of thousands of people bearing white candles chanted slogans calling for China to own up to the crackdown and release political dissidents. It was the largest commemoration on Chinese soil.

Organizers estimated the crowd at 150,000 — the largest rally since the first anniversary vigil in 1990 — while police put the number at 62,800.

"It is the dream of all Chinese people to have democracy!" the crowd gathered in Hong Kong's famous Victoria Park sang in unison.

A former British colony, the territory has retained its own legal system and open society since reverting to Chinese rule in 1997. Those killed in the violence were eulogized as heroes of the push for democracy in China, their names read aloud before the crowd observed a minute of silence in their memory.

"It's time for China to take responsibility for the killings," said Kin Cheung, a 17-year-old Hong Kong student. "They need to tell the truth."

On the mainland, government censors shut down social networking and image-sharing Web sites such as Twitter and Flickr and blacked out CNN and other foreign news channels each time they aired stories about Tiananmen.

Dissidents and families of crackdown victims were confined to their homes or forced to leave Beijing, part of sweeping efforts to prevent online debate or organized commemorations of the anniversary.

"We've been under 24-hour surveillance for a week and aren't able to leave home to mourn. It's totally inhuman," said Xu Jue, whose son was 22 when he was shot in the chest by soldiers and bled to death on June 4, 1989.

Officers and police cars were also stationed outside the home of Wang Yannan, the daughter of Zhao Ziyang, the Communist Party leader deposed for sympathizing with the pro-democracy protesters, according to the Hong Kong-based Information Center for Human Rights and Democracy. Wang heads an auction firm and has never been politically active.

In Hong Kong, tape recordings of Zhao recalling the violence of Tiananmen, used for his recently released posthumous memoir, were played during the rally.

In a further sign of the government's intransigence, the second most-wanted student leader from 1989 was forced to return to Taiwan on Thursday after flying to the Chinese territory of Macau the day before in an attempt to return home.

Wu'er Kaixi, in exile since fleeing China after the crackdown, told The Associated Press by phone he was held overnight at the Macau airport's detention center and that being denied entry on the Tiananmen anniversary was a "tragedy."

The student leader who topped the most-wanted list, Wang Dan, was jailed for seven years before being expelled to the United States in 1998.

In Washington, U.S. Secretary of State Clinton said in a statement Wednesday that China, as an emerging global power, "should examine openly the darker events of its past and provide a public accounting of those killed, detained or missing, both to learn and to heal."

Taiwanese President Ma Ying-jeou urged China to lift the taboo on discussing the crackdown.

"This painful chapter in history must be faced. Pretending it never happened is not an option," Ma said in a statement issued Thursday.

"We urge the U.S. to put aside its political prejudice and correct its wrongdoing and refrain from disrupting or undermining bilateral relations," Qin said in response to a question at a regularly scheduled news briefing. Qin refused to comment on the security measures — or even acknowledge they were in place.

"Today is like any other day, stable," he said.

Beijing has never allowed an independent investigation into the military's crushing of the protests, in which possibly thousands of students, activists and ordinary citizens were killed. Young Chinese know little about the events, having grown up in a generation that has largely eschewed politics in favor of raw nationalism, wealth acquisition and individual pursuits.

Authorities tightened surveillance of China's dissident community ahead of the anniversary, with some leading writers under close watch or house arrest for months.

Every year, about 7 million visitors explore Beijing's huge public square, Tiananmen. China has tried to erase the memory of the violent events there in 1989, but eyewitness stories, photos and videos still tell the tale. On the event's 20th anniversary, we take look at what happened that day, June 4, 1989, and since.

Background: Tiananmen Square dates back to the Ming Dynasty when Tianan Gate was originally erected. The square has been the setting for other protests, including one in 1919 and another in 1976.

What's there? The square is surrounded by a museum, a mausoluem, a hall and the great Tianan Gate. In the center of the square sits the giant stone marker. The square measures around 100 acres, making it the largest open square in the world.

What happened? In April 1989, a former leader of Chinese Communist Party died. Hu Yaobang was forced to resign because of his democratic leanings. On his death, a contingent of students and intellectuals gathered to demand that the party reassess his legacy. That gathering turned into a protest calling for dramatic and democratic reforms.

The protest: In Beijing, what began as largely a student protest grew to an estimated 1 million Chinese on the streets. A statue that largely resembles the Statue of Liberty was erected and several thousand protesters began a second form of civil disobedience, a hunger strike.

China's reaction: Armed troops and tanks entered the square, and shooting began. China said 241 people died. American media said it was as many as 800. Chinese Red Cross said 2,600; Amnesty International 1,000. A NATO figure put total dead among students and soldiers at 7,000.

"Tank Man": The still unidentified man became a symbol of the clash with his simple act of bravery.

"June 4 incident": That's what it's called in China. References to the protest had been removed from literature, and the Chinese government continues to censor Tiananmen Square information. Ahead of the anniversary, it also blocked access to Twitter, Flickr, Hotmail and some blogging Web sites.